Digital color reproduction printing systems typically include digital front-end processors, digital color printers, and post finishing systems (e.g., glosser system, binding system). These systems reproduce original color onto substrates (such as paper). The digital front-end processes take input electronic files (such as PDF or postscript files) composed of imaging commands and images from other input devices (e.g., a scanner, a digital camera) together with its own internal other function processes (e.g., raster image processor, image positioning processor, image manipulation processor, color processor, image storage processor, substrate processor, etc) to rasterize the input electronic files into proper image bitmaps for the printer to print. An operator may be assisted to set up parameters such as layout, font, color, paper, post-finishing, etc. among those digital front-end processes. The printer (e.g., an electrographic printer) takes the rasterized bitmap and renders the bitmap into a form that can control the printing process from the exposure device to writing the image onto paper. The post-finishing system finalizes the prints by adding finishing touches such as protection, glossing, and binding etc.
In an electrophotographic modular printing machine of known type, for example, the Eastman Kodak NexPress 2100 printer manufactured by Eastman Kodak, Inc., of Rochester, N.Y., color toner images are made sequentially in a plurality of color imaging modules arranged in tandem, and the toner images (also referred to as Dry Ink images) are successively electrostatically transferred to a receiver member adhered to a transport web moving through the modules. Commercial machines of this type typically employ intermediate transfer members in the respective modules for the transfer to the receiver member of individual color separation toner images. In other printers, each color separation toner image is directly transferred to a receiver member.
Electrophotographic printers having multicolor capability are known to also provide an additional toner depositing assembly for depositing specialized toners such as clear toner, pearlescent toner, metallic toner or lightly tinted toner. The purpose and use of these specialized toners depends upon the customer needs and can include gloss control, watermarking, security printing, highlighting with sheen and many other effects that go far beyond the original introduction of only clear toner for gloss effects and image overcoating.
Currently, there are methods for calibrating color toners such as cyan, magenta, yellow or black. These methods typically deposit the color toner onto a white substrate and then measure color response such as CIELAB or density using, for example, a spectrophotometer or reflection densitometer respectively. Some color responses, such as CIELAB, are more useful for relating to human color perception other color responses, such as XYZ tristimulus or Status A densitometry can work well for process control of color output. Process control refers to the control of the CMYK color toner laydown in an electrophotographic printer to achieve a consistent output color profile (Kuo et. al, U.S. Pat. No. 7,777,915).
A refinement of the traditional process control of the subtractive CMYK color toner was the introduction of additional color profiles to account for the effect of overcoat clear toner on the fusing process of the underlying color toner. In order to get more consistent color results for the CMYK images, U.S. Pat. No. 7,324,240 introduced the use of two color profiles one for the CMYK toner regions where clear toner is present and a separate color profile for the CMYK toner regions where there is no clear toner present. This patent found that the presence of clear toner on top of the CMYK toner changes the flow of the color toner below when it melts in the fusing step. Consequently, the color below the clear toner is not the same as the color of other portions of the substrate that do not have clear toner on top. While U.S. Pat. No. 7,324,240 also provides for multiple CMYK color profiles for different laydowns of clear toner, U.S. Pat. No. 7,324,240 does not provide a means for measuring and controlling the laydown of the clear toner.
U.S. Pat. No. 8,340,542 does provide a means for controlling the laydown of a clear toner layer provided to enhance transfer by measuring the scattering effect of the unfused clear toner on a an intermediate transfer surface. However, U.S. Pat. No. 8,340,542 does not provide a means for measuring the laydown of the clear toner in the image, nor does U.S. Pat. No. 8,340,542 account for the effect of fusing in the final print appearance that can only be detected by measuring the color signal of the toner (clear or CMYK) after fusing.
Color measurement after fusing of clear toner or other specialty toners such as white, clear, pearlescent, metallic and fluorescence toner deposited directly onto a receiver is very difficult because of the very low contrast of the clear toner compared to the white paper where no clear toner is present. But the increasing customer interest in features that are created using specialty toners now makes it necessary to apply process control to the specialty toner channel in order to maintain consistent high quality.
Thus, a robust and easy means for measuring the laydown of clear toner on an image or substrate after fusing is currently lacking and is provided in this invention.